This invention relates to a device for electrical discharge machining wherein the work surface is etched by electrical discharge machining to a predetermined surface roughness for providing the satinized indented surface on the work.
Heretofore, in providing a satin finish to a roll used for rolling a steel skelp, especially to a cold working roll, hard metal particles known as shots or grits are projected onto a ground roll surface for providing indented marks on the roll surface. In recent years, this type of machining is tentatively performed by an electrical discharge machining. The electrical discharge machining as used in this method consists essentially in providing an insulating liquid such as kerosene in the narrow discharge gap between the electrode and the work surface and a pulsed voltage is applied across the electrode and the work to cause an electrical discharge for thereby etching or machining the work surface. Such discharge is caused to occur repeatedly between the work in the form of a roll (hereafter abbreviated occasionally to roll) and said electrode while the roll is rotated about its own axis and moved longitudinally along the rotary axis of the roll for providing a satin finish on the roll surface consisting of spirally extending continuous discharge marks. This is the method for providing a uniform satin finish on the roll surface by utilizing an electrical discharge machining. The satinized roll surface thus obtained has many advantages over that obtained through projection of metal particles in that the indents are more accentuated and trim in shape while not being affected by the degree of hardness or the manufacture process of the roll, and in that the metal structure at or near the roll surface may be improved in toughness due to electrical discharge and thus may be most convenient for a roll used in the roll working.
According to the conventional practice for such electrical discharge dull machining, multi-head and multi-divided electrodes are used with a view to reducing the working time. However, a certain limit is imposed on the number of electrode segments obtained by such division of the electrode and there is moreover an inconvenience that the working time may be prolonged over that required in the conventional process in the region of low surface roughness on account of certain characteristics of electrical discharge machining.
In order to solve this problem, it is basically necessary to increase the number of the heads and the number of the working electrodes. So far, this has not been feasible because of the structural constraint inherent to the electric discharge machining device.